onlyif: Conditionally apply a function to an argument
In zatonovo/lambda.tools: Tools for Modeling Data with Functional Programming

Description Arguments Value Usage Details Note See Also Examples

This function conditionally applies a function to an argument given a logical condition.

`condition`	Logical statement used to conditionally apply fn to x
`fn`	A function to apply to x
`expr`	An expression
`x`	An object

Either expr if condition is true, otherwise x.

onlyif(condition, fn, x)

This function can be used to apply a function to a vector containing elements that lie outside the valid domain of fn. The function onlyif differs from ifelse in the sense that it is not vectorized and a closure can be used. For example,

ifelse(length(x) < 10, pad(x, 10 - length(x)), x)

yields the wrong result due to the length of the first argument. The onlyif function is designed for these situations.

onlyif(length(x) < 10, function(x) pad(x, 10 - length(x)), x).

Note that a closure is only required if an expression cannot be evaluated under both a TRUE or FALSE scenario.

The alternative would be to use a conditional block, which can result in improperly scoped code if one is careless.

The interface for this function is experimental. I'm looking for a way to preserve unevaluated expressions. Until then, I don't recommend using the function.

use_default

x <- 1:5
onlyif(length(x) < 10, pad(x, 10 - length(x)), x)
onlyif(length(x) < 10, function(x) pad(x, 10 - length(x)), x)

# This returns x
x <- 1:20
onlyif(length(x) < 10, function(x) pad(x, 10 - length(x)), x)